Recently aluminum alloys have increasingly been used widely as materials for structural sections for buildings and transport vehicles because they are light in weight and beautiful in appearance, have a strength and a corrosion resistance sufficient to permit application as materials for structural sections, and furthermore, are excellent in workability. A structural section of aluminum alloy is usually manufactured by extrusion so as to make the fullest possible use of properties thereof. When using an aluminum alloy as a material, manufacture into extruded sections permits expectation of reduced time and labor, an improved material yield and a higher dimensional accuracy than manufacture into rolled sections.
Conventionally, aluminum-magnesium-silicon alloys (hereinafter referred to as "Al-Mg-Si alloys") have long been used as corrosion resistant heat-treatable wrought aluminum alloys. Especially, an Al-Mg-Si alloy containing from 0.45 to 0.9 wt.% Mg and from 0.2 to 0.6 wt.% Si, i.e., JIS (the abbreviation of the "Japanese Industrial Standards") 6063 aluminum alloy is excellent not only in corrosion resistance but also in extrudability in particular. It is therefore a usual practice at present to manufacture most of extruded sections of aluminum alloy from said JIS 6063 alloy. However, said JIS 6063 alloy is defective in that it has a relatively low strength and is not provided with a strength required when using as a material for structural sections, i.e., a strength after a heat treatment of from 25 to 40 kg/mm.sup.2, particularly from 30 to 40 kg/mm.sup.2 (the strength of from 25 to 40 kg/mm.sup.2 is hereinafter referred to as the "medium strength").
From among Al-Mg-Si alloys, JIS 6061 aluminum alloy containing from 0.8 to 1.2 wt.% Mg and from 0.4 to 0.8 wt.% Si, and AA (the abbreviation of the "Standards of the Aluminum Association of America") 6070 aluminum alloy containing from 0.5 to 1.2 wt.% Mg and from 1.0 to 1.7 wt.% Si are known and applied in addition to said JIS 6063 alloy. These two alloys, which have a far higher strength after a heat treatment than said JIS 6063 alloy and a satisfactory corrosion resistance, although slightly inferior to that of said JIS 6063 alloy, are suitable for use as materials for structural sections requiring the medium strength, whereas they are defective in the workability, especially in extrudability. Therefore, said two alloys are problematic because of the too low productivity in extrusion as well as of the production of defects caused by cracks in working.
In the Registration Record of Aluminum association Alloy Designations and Chemical Composition Limits for Wrought Aluminum Alloys (revised as of June 1, 1974) published by the Aluminum Association of America, on pages 5 through 7, of which a copy is attached hereto, there are disclosed AA 6011 aluminum alloy containing from 0.6 to 1.2 wt.% Mg, from 0.6 to 1.2 wt.% Si, up to 1.5 wt.% Zn, from 0.4 to 0.9 wt.% Cu, up to 0.3 wt.% Cr and up to 0.8 wt.% Mn; AA 6253 aluminum alloy containing from 1.0 to 1.5 wt.% Mg, from 0.45 to 0.98 wt.% Si, from 1.6 to 2.4 wt.% Zn, up to 0.1 wt.% Cu and from 0.15 to 0.35 wt.% Cr; and AA 7472 aluminum alloy containing from 0.9 to 1.5 wt.% Mg, up to 0.25 wt.% Si, from 1.3 to 1.9 wt.% Zn, up to 0.05 wt.% Cu and up to 0.05 wt.% Mn. These three alloys are similar to the alloy of the present invention described later in that they contain magnesium, silicon and zinc. However, all said three alloys have chemical compositions different from that of the aluminum alloy of the present invention. Furthermore, said AA 6011 alloy and said 6253 alloy are aluminum alloys exclusively used for special applications such as cladding materials for cladded sheets.
In the paper of Mr. R. Sudholter presented in the Metall (25 Jahrgang, Heft 3, March 1971) published by Metall-Verlag GmbH, on page 251, there are disclosed an aluminum alloy containing 1.1 wt.% Mg, 0.6 wt.% Si and 2 wt.% Zn and another aluminum alloy further additionally containing 0.15 wt.% Mn. These two alloys are also similar to the aluminum alloy of the present invention in that they contain magnesium, silicon and zinc. However, both said two alloys are aluminum alloys used for casting and are not provided with properties necessary for use in extrusion.
In addition, in the U.S. patent application Ser. No. 876,275 filed on Feb. 9, 1978, U.S. Pat. No. 4,169,728, there is described a corrosion resistant bright aluminum alloy for die casting, consisting of, in weight percentage:
Mg from 1.1 to 3.0%, PA1 Si from 0.2 to 1.2%, PA1 Zn from 0.5 to 2.5%, PA1 Fe from 0.2 to 1.5%, PA1 Mn from 0.3 to 1.2%, PA1 Cu from 0.1 to 0.3%, and, PA1 Ti from 0.05 to 0.30%, PA1 Cr from 0.05 to 0.50%, PA1 B from 0.01 to 0.05%, PA1 Zr from 0.05 to 0.30%, PA1 V from 0.05 to 0.30%, PA1 Co from 0.05 to 0.50%, PA1 Sb from 0.05 to 0.30%, PA1 Ni from 0.05 to 2.00%, and, PA1 Be from 0.001 to 0.005%. PA1 Magnesium from 0.45 to 0.98%, PA1 Silicon from 0.3 to 0.8%, PA1 Zinc from 0.5 to 0.25%, and, PA1 Copper from 0.04 to 0.30%, PA1 Zirconium from 0.04 to 0.25%, PA1 Chromium from 0.04 to 0.30%, and, PA1 Manganese from 0.04 to 0.25%; PA1 Titanium from 0.01 to 0.20%, and, PA1 Boron from 0.01 to 0.06%. PA1 Magnesium from 0.45 to 0.98%, PA1 Silicon from 0.3 to 0.8%, PA1 Zinc from 0.5 to 2.5%, and, PA1 Copper from 0.04 to 0.30%, PA1 Zirconium from 0.04 to 0.25%, PA1 Chromium from 0.04 to 0.30%, and, PA1 Manganese from 0.04 to 0.25%; PA1 Titanium from 0.01 to 0.20%, and, PA1 Boron from 0.01 to 0.06%.
the balance aluminum and incidental impurities; said alloy including a corrosion resistant bright aluminum alloy for die casting further additionally containing from 0.001 to 2.000% in weight precentage in total of at least one element selected from the group consisting of:
The above-mentioned alloy is also similar to the aluminum alloy of the present invention described later in that the former contains magnesium, silicon and zinc. However, said alloy is an aluminum alloy used for die casting and contains at least 1.1 wt.% Mg to prevent occurrence of casting cracks and iron in a relatively high percentage with a view to improving the strippability of the cast alloy, this resulting in a lower workability, especially a lower extrudability.
There is therefore an increasing demand for a corrosion resistant aluminum alloy excellent in workability, particularly in extrudability, which has a medium strength of from 25 to 40 kg/mm.sup.2, especially from 30 to 40 kg/mm.sup.2 required when using as a material for structural sections.